1. Field of the Invention
This invention relates generally to magnetic tunneling junction (MTJ) devices such as MRAMs and read-heads and more particularly to the use of a novel seed layer that allows the formation of a junction layer of superior physical properties.
2. Description of the Related Art
The magnetic tunneling junction device (MTJ device) is essentially a variable resistor in which the relative orientation of magnetic fields in an upper and lower very thin dielectric layer (the tunneling barrier layer) formed between those electrodes. As electrons pass through the upper electrode they are spin polarized by its magnetization direction. The probability of an electron tunneling through the intervening tunneling barrier layer then depends on the magnetization direction of the lower electrode. Because the tunneling probability is spin dependent, the current depends upon the relative orientation of the magnetizations of magnetic layers above and below the barrier layer. Most advantageously, one of the two magnetic layers (the pinned layer) in the MTJ has its magnetization fixed in direction, while the other layer (the free layer) has its magnetization free to move in response to an external stimulus. If the magnetization of the free layer is allowed to move continuously, as when it is acted on by a continuously varying external magnetic field, the device acts as a variable resistor and it can be used as a read-head. If the magnetization of the free layer is restricted to only two orientations relative to the fixed layer (parallel and anti-parallel), the first of which produces a low resistance (high tunneling probability) and the second of which produces a high resistance (low tunneling probability), then the device behaves as a switch, and it can be used for data storage and retrieval (a MRAS).
Magnetic tunneling junction devices are now being utilized as information storage elements in magnetic random access memories (MRAMs). Typically, when used as an information storage or memory device, magnetic fields produced by orthogonally intersecting current carrying lines (digit and bit lines) orient the magnetization of the free layer so that it is either parallel or anti-parallel to the pinned layer; at a later time a sensing current passed through the MTJ indicates if it is in a high (antiparallel) or low (parallel) resistance state.
When used as a read head, (called a TMR read head, or “tunneling magnetoresistive” read head) the free layer magnetization is moved by the influence of the external magnetic fields of a recorded medium, such as is produced by a moving hard disk or tape. As the free layer magnetization varies in direction, a sense current passing between the upper and lower electrodes and tunneling through the barrier layer feels a varying resistance and a varying voltage appears across the electrodes. This voltage, in turn, is interpreted by external circuitry and converted into a representation of the information stored in the medium.
Whether it is used as an MRAM or as a TMR read head, fabrication of a high quality MTJ device presents considerable difficulties due to the necessity of forming layers of extreme thinness. Sun et al. (U.S. Pat. No. 6,574,079) provides a particularly well written statement of some of these difficulties. First, to obtain effective spin polarization of the conduction electrons, the magnetization of the electrode layers must be strong. This is itself a problem, since the layers are exceptionally thin. Second, the resistance of the tunneling barrier layer is typically high, which results in a poor ratio of signal-to-noise (S/N) in read head applications. If the resistance of the barrier layer is lowered by excessively thinning that layer, then fabrication processes such as lapping the air-bearing surface of the read head can create shorts through the barrier layer. Sun et al. teach the formation of a thin barrier layer within a general configuration of the following form:
Ta/NiFe/CoFe/Barrier/CoFe/Ru/CoFe/PtMn/Ta with Ta/Cu/Ta used as top and bottom leads. In the configuration of Sun et al. shown above, Ta is a seed layer, NiFe/CoFe is a free layer, CoFe/Ru/CoFe is a pinned (fixed) layer, PtMn is a pinning layer and Ta is a protective capping layer. Sun et al. find that a preferred barrier layer within the given configuration is a layer of oxidized NiCr, ie NiCrOx. The barrier layer so formed yields a junction resistance, RA, (area of junction, A, times total resistance, R) of about: RA=6.6 Ωμm2.
Applicants have discovered other recent prior art (commercially produced) TMR read head configurations analogous to that taught by Sun et al., including:                Ta/NiFe/MnPt/CoFe(10%)/Ru/CoFe(50%)/Al/NOX/CoFe—NiFe(18%)/Ta        Ta/NiFe/IrMn/CoFe(16%)/Al/(4.5)Hf(1.5)/NOX/CoFe—NiFe(18%)/TaIn the above notation Al/NOX refers to an aluminum layer that has been naturally oxidized to form an insulating barrier layer. Al (4.5)Hf(1.5)/NOX refers to a 4.5 angstrom aluminum layer over which is deposited 1.5 angstroms of Hafnium, with the composite layer then being naturally oxidized. CoFe(16%) refers to a CoFe alloy with 16% Fe by number of atoms. In each of the above configurations NiFe is a buffer layer for growing the antiferromagnetic layer of MnPt or IrMn.        
Applicants have found that a tunneling junction with improved performance over those of the prior art, particularly those cited above, can be made using a novel seed layer and method of forming it. The tunneling junction so formed, which can also be used in either a TMR configuration or an MRAM configuration, is capable of producing a junction resistance of RA≅1 Ωμm2 and a GMR ratio, DR/R>10% and a dielectric breakdown voltage, Vb>0.5 volts.